Process
Capability study through CP and CPk measurement.

Process capability
and process performance is through the study of the statistical
measurements of Cp and Cpk.

Definitions
of Cp and cpk

Cp = Represent Process Capability. An indicator to the capability
of a particular process.Cpk = Represent the index of the Process Capability study.
A tool to measure the Cp value for the effect of non-centered distribution.

the
study of CP

A common Process
Capability measure, Cp indicates how well the process distribution
fits within its specification limits ( Upper and Lower limit ).
In the examples below, processes (a) has Cp greater than one, capable
for mass production (b) is equal to one and (c) is less than one.
Higher chances to fall out of tolerance.

The
problem with Cp is that it does not measures how well the process
distribution is, the process may have high Cp but distributed off
the center of its limits, which can result in many rejects. The
solution to this is the study of it Cpk.

The
study of Cpk

Cpk study is
to measures where the Cp value distributed between the upper and
lower limits. Cpk only considers the variation half that is closest
to the specification limits, as in the figure below.

Cp and Cpk
have to be studied together to give a measure of both the potential
and centering of the process distribution within the specification
limits.

The larger the index, the more stable the process is, the less likely
that any item will be outside the specification. CPK value of 1.33
is the industry standard of minimum acceptable level.

CPK
Value

Capability

Inspection
and action needed

Less
than 1

Incapable

100 % inspection
may be needed for the production batch. Process improvement
is highly recommended.